Ion exchange waveguides and methods of fabrication

ABSTRACT

A method for fabricating ion exchange waveguides, such as lithium niobate or lithium tantalate waveguides in optical modulators and other optical waveguide devices, utilizes pressurized annealing to further diffuse and limit exchange of the ions and includes ion exchanging the crystalline substrate with a source of ions and annealing the substrate by pressurizing a gas atmosphere containing the lithium niobate or lithium tantalate substrate above normal atmospheric pressure, heating the substrate to a temperature ranging from about 150 degrees Celsius to about 1000 degrees Celsius, maintaining pressure and temperature to effect greater ion diffusion and limit exchange, and cooling the structure to an ambient temperature at an appropriate ramp down rate. In another aspect of the invention a powder of the same chemical composition as the crystalline substrate is introduced into the anneal process chamber to limit the crystalline substrate from outgassing alkaline earth metal oxide during the anneal period. In yet another aspect of the invention an anneal container is provided that allows for crystalline substrates to be annealed in the presence of powder without contaminating the substrate with the powder during the anneal process. Waveguides manufactured in accordance with the method exhibit superior drift performance.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of co-pending U.S. patent applicationSer. No. 09/309,361, filed May 11, 1999—now allowed—and entitled,“Method for Pressurized Annealing of Lithium Niobate and ResultingLithium Niobate Structures”, by Lee J. Burrows assignor to CaliforniaInstitute of Technology, a California non-profit corporation. Patentapplication Ser. No. 09/309,361—now allowed—claims the benefit ofprovisional patent application Ser. No. 60/084,940 filed May 11, 1998,and entitled “Pressurized Annealing of Lithium Niobate”. Thisapplication is also related to U.S. patent application Ser. No.09/418,725,—now U.S. Pat. No. 6,625,386—filed on even date herewithentitled “TITANIUM-INDIFFUSION WAVEGUIDES AND METHODS OF FABRICATION” byinventors Lee J. Burrows and William B. Bridges, both assignors to theCalifornia Institute of Technology; and U.S. patent application Ser. No.09/419,349,—now U.S. Pat. No. 6,567,598—filed on even date herewithentitled “TITANIUM-INDIFFUSION WAVEGUIDES” by inventor Lee J. Burrows,assignor the California Institute of Technology; and U.S. patentapplication Ser. No. 09/157,652,—now U.S. Pat. No. 6,518,078—filed Sep.21, 1998 entitled “ARTICLES USEFUL AS OPTICAL WAVEGUIDES AND METHOD FORMANUFACTURING SAME” by inventor Lee J. Burrows, assignor to theCalifornia Institute of Technology.

STATEMENT AS TO RIGHTS TO INVENTIONS

The United States Government has certain rights in this inventionpursuant to Grant No. F-19628-95-C-0002 awarded by the United States AirForce.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating ion exchangewaveguides in optical modulators using pressurized annealing and theresulting waveguides and modulators. More particularly, the presentinvention relates to a method for fabricating lithium niobate-ion orlithium tantalate-ion exchange waveguides using a pressurized oxygenatmosphere anneal process to further diffuse ions in the exchangeregion.

2. Background

Optoelectronic components can be fabricated on several types ofsubstrates including polymers, glass, semiconductors (e.g., galliumarsenide (GaAs) and indium phosphide (InP)) and inorganic materials(e.g., lithium niobate OLiNbO₃) and lithium tantalate (LiTaO₃)).Characteristically, an electro-optic material is one in which the indexof refraction changes with the application of an electric field. One ofthe most important components in optoelectronic systems is themodulator. Three competing technologies in this realm are: directmodulation of a semiconductor laser, semiconductor electro-absorptionmodulators, and the lithium niobate modulator. Currently, lithiumniobate modulators are the modulation devices of choice for many systemsbecause they yield high performance, are a relatively mature technologyand other modulation schemes impose limits not faced with lithiumniobate modulators.

Lithium niobate has proven to be a suitable medium for components suchas amplitude modulators, phase modulators, optical switches,polarization transformers, tunable filters and wavelength-selectiveoptical add/drop filters. Lithium niobate has also been used as the hostfor solid state lasers using rare earth ions, e.g., erbium Most currenttelecommunication and cable television system applications for LiNbO₃modulators involve discrete components for the optical transmittersubsystem. This configuration couples continuous wave lasers, typicallydiode-pumped YAG or erbium fiber oscillators, with lithium niobateexternal modulators and various wavelength and power stabilizationcomponents.

Lithium niobate is a popular nonlinear optical crystal for severalreasons including its large electro-optic coefficients, the ease withwhich high quality optical waveguides are fabricated and its amenabilityto conventional integrated circuit processing techniques. High qualityoptical waveguides are those that possess low loss and relatively highpower handling capabilities. Additionally, LiNbO₃ is a hard material,thus it is easily polished for fiber optical coupling which makes itsuse in optical network systems relatively uncomplicated. It is also arelatively inexpensive crystal, due in part to its long history of usein surface-acoustic-wave (SAW) filters for radio frequencies. Bycomparison, lithium tantalate LiTaO₃ is essentially interchangeable withlithium niobate as far as modulator characteristics are concerned, butthe use of LiTaO₃ is often cost prohibitive because it is not as widelycommercially used as LiNbO₃. Additionally, other optical crystallinestructures having the formula RMO3, where R is an alkaline earth metal,M is a Group IVB or Group VB metal, and O is oxygen, can also be used inthe fabrication of waveguides and modulators.

For example, one type of modulator incorporating the use of LiNbO₃ is aMach-Zehnder modulator. In a Mach-Zehnder modulator an incoming opticalbeam is split equally at a Y junction into two parallel waveguides,which then recombine at another Y junction after some distance.Electrodes are used to apply an electric field in the region of theoptical waveguides. The electric field causes a relative phase shiftbetween the two branches. If the phase shift in both arms is the same,then the two optical signals will recombine constructively at the secondY branch and no optical power will be lost. If there is a phasedifference between the two arms, then there will be destructiveinterference and some optical power will be lost The resultingdestructive and constructive interference causes the output opticalintensity to vary between a minimum and a maximum value.

In other electro-optic applications optical waveguide directionalcouplers can also be used as electro-optic modulators. In this type ofmodulator two waveguides are placed on the lithium niobate substrate invery close proximity to one another. The wave from one guide can “leak”into the other guide. By applying an electric field to the lithiumniobate, the amount of “leakage” can be controlled, thus modulating thetransfer of power from one guide to the other. Currently, differentcommercial application requirements favor either directional couplers orMach-Zehnder modulators.

The advance of high-speed, large bandwidth, digital and analogcommunications has led to a demand for the external modulatorconfiguration. The most common approach utilizes a low-noise, high powerdiode-pumped laser whose signal is sent to the LiNbO₃ modulator viaoptical fiber. The modulator applies either analog or digitalinformation to the optical carrier.

When using lithium niobate in the fabrication of optical waveguides andoptical modulators it is desired to avoid having a niobium-rich,lithium-poor and/or oxygen poor composition. When bulk lithium niobatehas such niobium rich-compositions, and is then processed at hightemperatures (T>300 degrees Celsius), growth of the LiNb₃O₈ phase in thecrystal may occur. This phase is undesirable because it is not opticallytransparent and leads to high losses in optical waveguides and opticalmodulators.

Such niobium-rich compositions can occur in two different manners duringfabrication of optical waveguides and optical modulators. First, typicalion exchange procedures result in the replacement of lithium atoms inthe crystal lattice with a diffusing ion leading to a lithium niobatecomposition relatively rich in niobium. Second, standard hightemperature (temperatures in excess of 300 degrees Celsius) processingof lithium niobate can cause Li₂O out-diffusion, and result inniobium-rich, and lithium and oxygen poor compositions.

To eliminate the undesired LiNb₃O₈ phase from forming in the crystal,high temperature processing, such as the stress relieving annealprocess, is usually performed in a wet atmosphere of inert carrier gas,such as nitrogen (N₂) or argon (Ar₂), or in a wet atmosphere of oxygen(O₂). This type of anneal process involves bubbling the inert carriergas or oxygen gas through water (H₂O). The wet atmosphere has beenconsidered beneficial in the past because the H₂O breaks down into H⁺and OH⁻ ions which chemically attack the LiNb₃O₈ phase and break it backdown into LiNbO₃. A typical wet atmosphere anneal operation is performedat a temperature of about 350 degrees Celsius for a period of 5 to 6hours in a wet, flowing environment. The present inventor has discoveredthat a drawback of this type of high temperature processing is that theH₂O gives off undesirable protons (H⁺) which are attracted by thelithium niobate and result in an inadvertent proton-exchanged surfacelayer occurring. These protons remain in the modulators afterfabrication and flow relatively freely among the waveguides, theelectrodes, the lithium niobate crystal and the buffer layer interface.It is now understood that these free flowing protons can and doadversely affect a modulator's DC-bias stability because they act ascharge carriers and are driven by the applied electric fields, causingthe response of the final product to drift over time when a bias isapplied.

Additionally, high temperature processing (>300 degrees Celsius) leadsto oxygen out-diffusion of lithium niobate structures. Thisout-diffusion tends to form a surface layer on a lithium niobatesubstrate or a lithium niobate waveguide that is oxygen poor incomposition. This oxygen poor region acts as a surface optical waveguideand causes undesirable leakage of light out of the lithium niobatesubstrate.

Ion exchange waveguides have typically been fabricated by treating orexchanging the surface of the crystalline substrate with a source ofions. In most applications the exchanging has been performed withprotons in the form of an acid, such as sulfuric acid or benzoic acid.The exchanging allows for the initial diffusion to take place, resultingin up to about 50% ion exchange (i.e., the ions replace the alkalineearth metal atoms in the crystal lattice). The initial exchangingprocess is then followed by a procedure that will further diffuse theions and drive the diffusion region farther into the depth of thecrystalline substrate. Ion exchange can also be accomplished by usingalkaline earth metal salt (i.e., lithium salt) as a buffer to theexchanging acid. Salt treatments are inefficient because they are timeprohibitive, some salt treatments can take upwards of 48 hours. Standardwet anneal processes introduce undesirable protons which are attractedby the crystalline substrate and result in an inadvertent protonexchanged surface layer occurring.

It would therefore be highly advantageous to devise a fabrication methodfor ion exchange waveguides that uses a new high temperature annealingprocess that inhibits both the formation of the undesirable LiNb₃O₈phase in the crystal and outdiffusion of O₂ without the processintroducing significant numbers of free flowing protons that will affectthe modulator's DC-bias stability.

BRIEF DESCRIPTION OF THE INVENTION

A method for fabricating ion exchange waveguides, such as lithiumniobate or lithium tantalate waveguides in optical modulators and otheroptical waveguide devices, utilizes pressurized annealing to furtherdiffuse and limit exchange of the ions and includes ion exchanging thecrystalline substrate with a source of ions and annealing the substrateby pressurizing a gas atmosphere containing the lithium niobate orlithium tantalate substrate above normal atmospheric pressure, heatingthe substrate to a temperature ranging from about 150 degrees Celsius toabout 1000 degrees Celsius, maintaining pressure and temperature toeffect greater ion diffusion and limit exchange, and cooling thestructure to an ambient temperature at an appropriate ramp down rate. Inanother aspect of the invention a powder of the same chemicalcomposition as the crystalline substrate is introduced into the annealprocess chamber to limit the crystalline substrate from outgassingalkaline earth metal oxide during the anneal period. In yet anotheraspect of the invention an anneal container is provided that allows forcrystalline substrates to be annealed in the presence of powder withoutcontaminating the substrate with the powder during the anneal process.Waveguides manufactured in accordance with the method exhibit superiordrift performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G are cross-sectional views of various stages in thefabrication of a lithium niobate ion exchange optical modulator thatincorporates pressurized oxygen atmosphere annealing in accordance witha one embodiment of the present invention.

FIG. 2A is a cross-sectional view of a container used for annealinglithium niobate structures in an environment having lithium niobatepowder in accordance with one embodiment of the present invention.

FIG. 2B is a cross-sectional view of a pressurizable vessel used forannealing lithium niobate in accordance with one embodiment of thepresent invention.

FIG. 3 is a process flow diagram of a process for fabricating structuresin accordance with a recently preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the inventionwill readily suggest themselves to such skilled persons from anexamination of the within disclosure.

In accordance with one presently preferred embodiment of the presentinvention a method is provided for fabricating an ion exchange lithiumniobate (LiNbO₃) optical modulator. Pressurized annealing is used tofurther diffuse ions in the waveguide as is detailed below. Ion exchangefabrication involves altering one or more refractive indices (e.g., theextraordinary and ordinary refractive indices) of the crystal substratein the region where the ion diffusion occurs to create an opticalwaveguide. The following exemplary description of the modulatorfabrication process is intended to limit the introduction of undesirablefree protons into the fabrication process. FIGS. 1A-1G are crosssectional views of a modulator after completion of selected steps in thefabrication process.

The present invention may also be defined as a method for fabricating anion exchange lithium tantalate (LiTaO₃) optical modulator. Lithiumtantalate is closely related in terms of electro-optical properties toLiNbO₃ but is not currently widely used in commercial electro-opticalmodulator applications because to do so would be cost prohibitive. Thesame or similar pressurized anneal process as described below for LiNbO₃would be employed in the annealing of LiTaO₃. Additionally, the crystalsubstrate that is used to form the optical modulator may be formed froma crystalline structure having the formula RMO₃, where R is an alkalineearth metal, M is a Group IVB or Group VB metal, and O is oxygen.

Referring to FIG. 1A, a modulator build 10 (a partially fabricatedmodulator) is shown after the formation of a mask layer 14 over theLiNbO₃ crystal substrate 12. The mask layer 14 may comprise siliconoxide (SiO₂), chromium oxide (Cr₂O₃), aluminum oxide (Al₂O₃) or anothersuitable masking layer material. Preferably, the mask material should berobust enough to withstand exposure to acids conceivably introduced inlater processing. The use of such masking layer materials are well knownby those of ordinary skill in the art. Mask layer 14 may be formed byusing conventional plasma deposition, sputtering or thermal evaporationtechniques, all of which are well known by those of ordinary skill inthe art.

In FIG. 1B, standard photolithography techniques are used to place aphoto resist layer 16 over the mask layer 14. As shown in FIG. 1B thephotolithography process will result in the formation of channels in thephotoresist in those areas where waveguides are to be fabricated. Theuse of such photolithography techniques are well known by those ofordinary skill in the art.

Referring to FIG. 1C a conventional plasma etch or reactive ion etch(RIE) technique is preferably then employed to form channels 18 throughthe mask layer 14 exposing the LiNbO₃ crystal substrate 12. The channelwidths can be of a dimension suitable for the intended wavelength of theconstrained wave. For example, for a 1.3 micron Optical Signal channelwidths of from about 5 microns to about 7 microns are presentlypreferred. A plasma etch or RIE procedure is preferably chosen so thatit selectively etches away the material chosen as mask layer 14. Itshould be noted that the etch process should preferably avoid wetetching. Wet etching generally involves the use of acids and thus wouldgenerally be likely (unless deuterated acids are used) to introduce freeprotons into the waveguide scheme that would adversely affect theLiNbO₃. As previously discussed, free protons in the waveguide tend toincrease the likelihood of output voltage drift over time.

Once the plasma etch or RE process is completed it may be desirable tostrip away residual photoresist. However, in some instances thephotoresist will be robust enough to withstand exposure to acidicmaterials and thus no stripping process needs to be employed. If anoptional stripping process is used it may be accomplished with an oxygenplasma or by using an acetone wash. The use of acids for strippingpurposes should be avoided as they have a tendency to introduce freeprotons into the modulator construct and these free protons increase thelikelihood of latent drift in the modulators.

Referring to FIG. 1D, shown is the modulator build 10 after undergoinginitial ion exchange diffusion. The exposed LiNbO₃ crystal substrate 12is treated with an acid, preferably a deuterated acid, such asdeuterated sulfuric acid (D₂SO₄), deuterated benzoic acid (C₇D₆O₂) oranother suitable deuterated acid. Deuterated sulfuric acid may beobtained in pure concentration from the Alfa Aesar Corporation of WardHill, Mass. and other vendors.

The use of deuterated acids in the diffusion process is beneficial forproviding stability and immobility to the crystal lattice structure. Thedeuterium atom has an additional neutron in the presence of a hydrogenatom, and has a mass approximately twice that of a normal hydrogen atom.Since deuterium ions are heavier they possess higher activation energiesand pose a lesser likelihood of moving out of the binding site than dothe lighter hydrogen ions. Additionally, deuterium ions are closer insize to the lithium atoms than hydrogen ions. Individual protons (H⁺)which differ in size and valence number from the replaced lithium atoms,have a tendency to impart more mobility to the crystal lattice. As aresult of this mobility, the optical waveguides and the refractive indextend to drift over time, which alters the output intensity of thewaveguide. However, when deuterium ions diffuse into the lithium niobatecrystal structure and replace lithium atoms they tend to “lock down” inthe crystal lattice and provide for more overall stability in thelattice. Note that tritium (H³) could advantageously be used instead ofdeuterium (H²) and should be considered as equivalent. Due to militaryrestrictions, however, tritium is not widely commercially available insignificant quantities.

The first initial diffusion occurs with ions replacing up to about 50%of the lithium atoms in the initial exchange region 20. The depth of theinitial ion diffusion region 20 will be dependant upon the cut of thecrystal, the optical wavelength and whether the waveguide is single-modeor multi-mode. For an optical modulator having Z-cut LiNbO₃ crystal, asingle mode and an optical wavelength of 1300 nanometers, the initialion exchange region 20 is typically about 0.4 microns in depth. Theinitial diffusion can be accomplished by immersing the modulator build10 in a vented ion exchange chamber having the appropriate acid at thedesired temperature. In general the ion exchange chamber will have theacid held at a temperature between about 160 degrees Celsius and about240 degrees Celsius. The diffusion soak time will last from about 5minutes to about 270 minutes. The temperature and time of the initialdiffusion process will be dependant upon the acid used and the abilityto achieve about 35% to about 50% lithium exchange to a depth of about0.4 microns in LiNbO₃ crystal substrate 12. The diffusion coefficientwill vary dependant upon the pH of the acid used, the temperature of theacid bath and the duration of the diffusion soak.

Note that after initial ion exchange and prior to anneal the percentageof Lithium atoms in the waveguide area exchanged for deuterons will beon the order of about 50%. After anneal the percentage will be loweredto about 12% but cover a larger volume. Note that beneficial effectsoccur with concentrations of deuterons in excess of about 1%.

A more complete discussion of how diffusion depths and percentages arecalculated in relation to time, temperature and pH is omitted from thisdisclosure in order to avoid overcomplicating the disclosure. See, forexample, co-pending U.S. patent application Ser. No. 09/157,652 filed onSep. 21, 1998, in the name of inventor Lee J. Burrows, entitled“Articles Useful as Optical Waveguides and Method for ManufacturingSame” for a disclosure of an appropriate discussion of calculatingdiffusion depths and percentages. That disclosure is hereby expresslyincorporated herein by reference as if set forth fully herein.

Once the initial diffusion process is completed, the photoresist layer16 and the mask layer 14 may be removed by subjecting modulator build 10to a suitable stripping procedure. A suitable stripping process may beaccomplished with techniques well known in the art such as oxygen plasmaetching or by using an acetone wash. The use of acids for strippingpurposes should be avoided as they have a tendency to introduce freeprotons into the modulator construct and these free protons increase thelikelihood of latent drift in the modulators. This stripping proceduremay be performed after the initial diffusion soak or after thesubsequent anneal process. After the initial diffusion soak or, ifwarranted, after the photo resist stripping process residual acids maybe washed off with a suitable solvent such as propenyl or acetone.

Following the initial diffusion process the waveguide undergoes apressurized anneal process that serves to further diffuse the ionexchange and drive the ions into greater depths within LiNbO₃ crystalsubstrate 12. As shown in FIG. 1E, the resulting pressurized annealprocess will result in the ions having an ion exchange region 22 with apenetration depth of about 6 microns and less than or equal to about 12%of the lithium atoms in ion exchange region 22 will have been replacedwith ions. The resulting ion exchange region 22 has one or morerefractive indices that differ from the refractive index of theuntreated LiNbO₃ crystal substrate 12. Preferably ion exchange region 22exhibits no decrease to a slight increase in extraordinary refractiveindex relative to the bulk of the untreated LiNbO₃ crystal substrate 12.Ion exchange region 22 is capable of constraining a propagating wavesuch that the wave propagates through ion exchange region 22 and is notscattered or diffused through the bulk of the untreated LNbO₃ crystalsubstrate 12. Preferably modulator build 10 exhibits the property ofallowing a wave propagating through the build to be modulated by anexternal force.

In accordance with another embodiment of the present invention, prior tothe anneal process the modulator build 10 may be placed in a containerthat allows for lithium niobate powder to be placed in close proximityto lithium niobate substrate 12. Within the container the modulatorbuild 10 and the lithium niobate powder are isolated by a porousinterface that allows gas to flow between the lithium niobate structureand the lithium niobate powder but does not allow for the lithiumniobate powder to contaminate modulator build 10. The container isclosed by slip fit caps that allow for oxygen gas to enter into thecontainer when a pressure differential exists yet restricts the outwardflow of lithium oxide (Li₂O) in the absence of a pressure differential.

The purpose of the lithium niobate powder is to induce lithium oxide(Li₂O) outgassing in the lithium powder during the anneal process whileretarding the same outgassing in LiNbO₃ crystal substrate 12. Aspreviously discussed lithium niobate will outgas Li₂O when exposed totemperatures in excess of 300 degrees Celsius. The outgassing leads to alithium niobate substrate that is poor in lithium. Lithium poorstructures are prone to the LiNb₃O₈ phase forming in the crystal. TheLiNb₃O₈ phase is not optically transparent and causes high losses inwaveguides. The lithium niobate powder has a much larger surface areabeing a collection of granules rather than a monolithic structure and isgenerally more reactive than LiNbO₃ crystal substrate 12; thereforeoutgassing will occur more readily in the lithium niobate powder. Oncethe anneal environment is saturated with Li₂O outgassed from the powder,LiNbO₃ crystal substrate 12 is less likely to outgas Li₂O.

Shown in FIG. 2A is a cross-sectional illustration of such an annealingcontainer 100 having both a modulator build and lithium niobate powdercontained within. This illustration is shown by way of example, othercontainers that meet this intent and purpose are also feasible andwithin the inventive concepts herein disclosed. The container 100 is atube-like structure that may be fabricated from a high temperatureceramic material such as aluminum oxide (Al₂O₃). The modulator build 102is placed in the center region of the tube and two porous plugs 104 arepositioned within the tube a short distance from the tube endings. Theporous plugs 104 may comprise any high-temperature material such asAl₂O₃ or a similar material. By way of example, the porosity of porousplugs 104 may be defined by the material having a plurality of holes ofdiameter on the order of about 20 microns. Plugs 104 allow for therelatively free flow of O₂ and Li₂O gases between the chamber 106housing the LNbO₃ crystal and the chamber(s) 108 housing the lithiumniobate powder. Once the plugs 104 are positioned within the container100, lithium niobate powder 110 is placed in one end or both ends of theplugged container 100. The amount of lithium niobate powder used will bedependant on the internal area of the sealable container. By way ofexample, the amount of the lithium niobate powder may be 2.5 grams for atube having a volume of about 1.0 in³ to about 2.0 in³. Once the lithiumniobate powder 110 has been properly positioned in the tube, the tube iscapped at both ends with loose slip fit caps 112 that are typicallyformed from the same high temperature material as container 100. Slipfit caps 112 will allow oxygen gas to enter container 100 when apressure differential exists yet restrict the outward flow of Li₂Oduring the anneal process when the environment is pressure normalized(i.e. no significant pressure gradient exists).

Additionally, other anneal containers can also be configured. Forexample, an anneal container having one chamber or region for containingmodulator build 102, a second region containing lithium niobate powder110 and a porous wall or plug 104 separating the two regions is alsofeasible and within the inventive concepts herein disclosed.

Once the container of FIG. 2A is properly assembled it can be placedwithin a sealable and pressurizable vessel 120. A cross-sectional viewof such a vessel is shown in FIG. 2B, enclosing the container 100 ofFIG. 2A. The pressurizable vessel 120 is typically formed from a metalmaterial such as stainless steel or a quartz or ceramic tube s withpressure fittings on it. In this illustration vessel 120 is tube-like instructure and has fittings 122 and 124 at opposite ends of vessel 120.Fitting 122 is a fixed fitting and fitting 124 allows for vessel 120 tobe vacuum pumped and pressurized with oxygen gas.

Additionally, annealing with lithium niobate powder can be undertakenwithout the use of the container, such as the one shown in FIG. 2A. Itis also possible and within the inventive concept herein disclosed toplace the lithium niobate powder directly in pressurized vessel 120.However, the quantity of required lithium niobate powder increasessubstantially when the powder is placed directly inside the pressurizedvessel, making this alternate embodiment, in most instances, morecostly.

The anneal process begins by placing modulator build 100 in aconventional annealing oven. Any suitable oven can be used as theannealing chamber and the use of such ovens are widely known by those ofordinary skill in the art. It is possible to use an anneal oven that haspressurizing capabilities in which case the use of the separatepressurizable vessel of FIG. 2B would be unnecessary. Upon placing thelithium niobate structure into the oven, the oven or pressurizablevessel is sealed and then the oven or pressurizable vessel is vacuumpumped down to approximately 100 microns pressure or less to eliminatecontaminants from the annealing environment. The vacuum pump downprocedure is optional and in some instances the need to removecontaminants from the annealing environment may not be of concern. Ifthe slip fit caps 112 do not make a good seal to the container 100 or ifthe plugs 104 do not make a good fit with the inside of container 100,creating a pressure differential across them may have the undesirableresult of causing turbulence which deposits some powder 110 on chip 102.To avoid this, the vacuum step may be avoided and pressurized gas blownthrough the oven from port 124 (FIG. 2B) to port 122 (FIG. 2B) toeliminate most important contaminants without causing movement of powder110. Once the oven or pressurizable vessel has been sealed andoptionally vacuumed it is then pressurized with oxygen gas (O₂). Thisoxygen need not be particularly pure and industrial or cutting gradeoxygen as used with acetylene torches will suffice. The pressurizedoxygen atmosphere serves to prevent oxygen outgassing. It should benoted that the atmosphere is a pure oxygen gas environment, no H₂O ispresent and thus no free protons or other radicals are given off thatwould adversely affect the LiNbO₃. The pressure in the O₂ atmosphere maybe raised to just above ambient atmospheric pressure to about 250 psiabove ambient atmospheric pressure. An optimal anneal pressure range isfrom about 1 psi to about 25 psi above ambient atmospheric pressure,preferably about 6 psi above ambient atmospheric pressure. Applicationshave shown that annealing at pressures above 25 psi tends to cause theLiNbO₃ to turn green in color. While the green discoloration does notappear to affect the structure negatively, to avoid this discolorationpressures below 25 psi above ambient atmospheric pressure should beused.

While oxygen (O₂) is presently preferred and provides the best results,the following gasses may also be used with slight reduction inperformance: Nitrogen (N₂), Argon (Ar), Helium (He). Purities on thesegasses may also be the same as for O₂, i.e., industrial purities.

Once the oxygen pressurization has been implemented in the annealchamber the temperature in the oven is then raised to the appropriatelevel that affects the necessary degree of ion exchange or stress reliefdesired. In general, the temperature can be raised to about 100 degreesCelsius to about 600 degrees Celsius. A temperature of about 300 degreesCelsius will generally allow for the necessary further ion diffusioninto required depths of the lithium niobate crystal substrate.Temperatures above 1000 degrees Celsius are generally undesirablebecause they allow for further undesirable phase changes to occur in theLiNbO₃ crystal. A preferable anneal temperature is about 300 degreesCelsius. It is generally advisable to anneal at lower temperatures tocontrol the depth of ion exchange. However, higher temperature annealsat this stage can be conducted and would decrease the anneal timeaccordingly. The ramp up rate for elevating the temperature in the ovenmay be in the range of about 0.5 degrees Celsius per minute to about20.0 degrees Celsius per minute. The preferred ramp up rate is 10degrees Celsius per minute.

The required duration of the anneal process will depend upon thetemperature at which the annealing takes place. For a higher temperatureanneal process a shorter anneal period is required and for a lowertemperature anneal process a longer anneal period is required. Theanneal period is measured from the time at which the desired elevatedtemperature is reached. In general, the anneal process will last fromabout 4 hours to about 8 hours. Longer anneal times are possible but arenot commercially acceptable. Shorter anneal times are possible but willrequire higher anneal temperatures. The desired elevated temperature andelevated pressure should be maintained throughout the duration of theanneal process. Preferably the anneal process will last approximately 6hours.

The anneal process is completed by cooling the lithium niobate structurein a rapid manner. The ramp down rate for the crystal structure may befrom about 0.5 degrees Celsius per minute to about 40 degrees Celsiusper minute. A presently preferable ramp down rate for the crystalstructure is 20 degrees Celsius per minute. A degrees per minute rampdown rate can be achieved by opening the anneal oven to a roomenvironment while continuing to blow oxygen across the surface ofmodulator build. Faster ramp down rates decrease the likelihood ofLiNb₃O₈ forming during the cool down process.

As shown in FIG. 1E, the resulting presently preferred pressurizedanneal process will result in the ions having an ion exchange region 22with a penetration depth of about 6 microns and less than or equal toabout 12% of the lithium atoms in ion exchange region 22 will have beenreplaced with ions.

As shown in FIG. 1F , once the secondary anneal process is completed andafter a strip operation has removed mask layer 14 and photo resist layer16, a buffer layer 24 is placed over the entire modulator build 10.Buffer layer 24 serves as insulation and is generally about 500 to about10000 angstroms in thickness, preferably about 3000 angstroms. Bufferlayer 24 may comprise silicon oxide (SiO₂), silicon nitride (Si₃N₄ orother phases), a combination of silicon oxide and silicon nitride,indium tin oxide (ITO) or another suitable insulator material. Prior toplacing buffer layer 24 over the waveguides and the crystal substrate astrip operation may be employed to remove the waveguide template (i.e.mask layer 14 and buffer layer 16). The strip operation can beaccomplished using an oxygen plasma, an acetone wash, PVD etch oranother suitable technique that does not introduce the use of acids intothe fabrication process. A conventional deposition technique, such assputtering, may be used to place buffer layer 24 over modulator build10. It should be noted that the use chemical vapor deposition (CVD)techniques should be avoided as such processing tends to generateundesirable free protons.

Referring to FIG. 1G, the modulator build 10 is completed by placingelectrodes 26 above the buffer layer 24. As shown, electrodes 26 can beplaced directly above waveguide regions 22 or they may be placed atother locations atop buffer layer 24. As is known by those of ordinaryskill in the art, Z orientation modulators will have electrodes formeddirectly above the waveguides and X or Y orientation modulators willhave electrodes formed offset from the waveguides. Standard depositiontechniques are used to form electrodes 26 above buffer layer 24. Astandard deposition technique may include a photoresist process, aconventional plasma deposition, sputtering or thermal evaporationprocess, a plasma etch process and a strip and acetone or propenyl washprocess to eliminate the photoresist The electrodes may be formed fromgold (Au), chromium gold, titanium and gold or other suitable electrodematerials. The use of chromium and titanium in combination with gold oras a pure thin layer between buffer layer 24 and electrode 26 is for thepurpose of increasing adhesion between the gold and the buffer layer.

Once the electrodes have been fabricated the modulator build 10undergoes a post build anneal process to relieve stress in the crystal,the electrodes and/or the buffer layer. In some instances, this annealprocess may also preferably be performed after the formation of bufferlayer 24. This pressurized anneal process is referred to as the primaryanneal process that all modulator builds undertake as part of standardmodulator fabrication. Lithium niobate has inherently high stresscoefficients and relieving stress is essential to assure that outputdrift does not occur in the modulators. This pressurized anneal processhas been discussed in detail above. The preferred anneal temperature isabout 300 degrees Celsius. The preferred anneal atmosphere is oxygen(O₂) at about 6 psi above ambient atmospheric pressure although a rangeof pressure above ambient from about 1 psi to about 25 psi above ambientatmospheric pressure will work. The preferred anneal time is about 4hours to about 6 hours. The anneal process may incorporate the use ofpowdered lithium niobate to lessen the likelihood of modulator build 10outgassing Li₂O.

FIG. 3 summarizes the fabrication procedures detailed above. Atreference number 120 the substrate is etched while avoiding theintroduction of free protons. At reference number 122 ions (preferablydeuteruted ions) are introduced into the etched regions of thesubstrate. At reference number 124 the ions are diffused into thesubstrate. At reference number 126 the substrate is placed into apassive vessel. At reference number 128 the pressure vessel ispressurized (preferable with O₂ ). At reference number 130 the substratetemperature is ramped up to the maximum anneal temperature. At referencenumber 132 the substrate is held at maximum anneal temperature andpressure for the duration of the anneal period. At reference number 134the substrate is cooled at a ramp down rate back to ambient. Atreference number 136 the vessel is depressurized and the substrateremoved. At reference number 138 electrode(s) are optionally formed onthe surface of the substrate. An additional anneal procedure may alsothen be performed.

Alternative Embodiments

Although illustrative presently preferred embodiments and applicationsof this invention are shown and described herein, many variations andmodifications are possible which remain within the concept, scope andspirit of the invention, and these variations would become clear tothose skilled in the art after a perusal of this application. Theinvention, therefore, is not limited except in spirit of the appendedclaims.

What is claimed is:
 1. A method for annealing a lithium niobatesubstrate, the method comprising: heating said lithium niobate substratein an environment having lithium niobate powder disposed therein, thelithium niobate substrate being separated from the lithium niobatepowder by a barrier allowing a free flow of gas within the environment,wherein the barrier allows for the free flow of gas between the lithiumniobate substrate and powder.
 2. A method for annealing lithium niobate(LiNbO₃) structures, the method comprising: heating a lithium niobatestructure to a maximum anneal temperature in an oxygen gas (O₂)environment having lithium niobate powder disposed therein, the lithiumniobate substrate being separated from the lithium niobate powder by abarrier, the barrier inhibiting movement of the powder thereacross whilepermitting a free flow of the oxygen gas within the environment;pressurizing the oxygen gas atmosphere to exceed ambient atmosphericpressure; maintaining temperature and pressure for a period of at leastabout 4 hours; and cooling the structure to ambient temperature.
 3. Themethod of claim 2 wherein said maximum anneal temperature is in a rangeof about 100° C. to about 1000° C.
 4. The method of claim 2 wherein saidmaximum anneal temperature is in a range of about 300 degrees Celsius toabout 600 degrees Celsius.
 5. The method of claim 2 wherein said maximumanneal temperature is about 300 degrees Celsius.
 6. The method of claim2 wherein said pressurizing includes pressurizing the oxygen gasatmosphere to a pressure within a range of about 1 psi above ambientatmospheric pressure to about 25 psi above ambient atmospheric pressure.7. The method of claim 2 wherein said pressurizing includes pressurizingthe oxygen gas atmosphere to a pressure of about 6 psi above ambientatmospheric pressure.
 8. The method of claim 2 wherein said coolingincludes cooling the substrate from the maximum anneal temperature at arate within a range of rates of about 0.5 degrees Celsius per minute toabout 40 degrees Celsius per minute.
 9. The method of claim 2 whereinthe barrier is a porous interface.
 10. The method of claim 2 whereinsaid cooling includes cooling the substrate from the maximum annealtemperature at a rate of about 20 degrees Celsius per minute.
 11. Amethod for annealing lithium niobate (LiNbO₃) structures, the methodcomprising: heating a lithium niobate structure in an oxygen gas (O₂)and lithium niobate powder environment to a maximum anneal temperature;pressurizing the sealed oxygen gas atmosphere to exceed ambientatmospheric pressure; maintaining temperature and pressure for a periodof at least about 4 hours; and cooling the structure to ambienttemperature, said cooling including: cooling the substrate from themaximum anneal temperature at a rate of about 20 degrees Celsius perminute.
 12. A method for annealing a lithium tantalate substrate, themethod comprising: heating said lithium tantalate substrate in anenvironment having lithium tantalate powder disposed therein, thelithium tantalate substrate being separated from the lithium tantalatepowder by a barrier, the barrier inhibiting movement of the powderthereacross while permitting a free flow of gas within the environment.13. A method for annealing lithium tantalate (LiTaO₃) structures, themethod comprising: heating a lithium tantalate structure to a maximumanneal temperature in a gas environment having lithium tantalate powderdisposed therein, the lithium tantalate substrate being separated fromthe lithium tantalate powder by a barrier, the barrier inhibitingmovement of the powder thereacross while permitting a free flow of gaswithin the environment; pressurizing the gas atmosphere to exceedambient atmospheric pressure; maintaining temperature and pressure for aperiod of at least about 4 hours; and cooling the structure to ambienttemperature.
 14. The method of claim 13 wherein said maximum annealtemperature is in a range of about 100° C. to about 1000° C.
 15. Themethod of claim 13 wherein said maximum anneal temperature is in a rangeof about 100 degrees Celsius to about 600 degrees Celsius.
 16. Themethod of claim 13 wherein said maximum anneal temperature is about 300degrees Celsius.
 17. The method of claim 13 wherein said gas is oxygen(O₂).
 18. The method of claim 13 wherein said gas is one or more gassesselected from the group consisting of Nitrogen (N₂), Argon (Ar), Helium(He) and Oxygen (O₂).
 19. The method of claim 13 wherein the barrier isa porous interface.
 20. The method of claim 13 wherein said pressurizingis to within a pressure range of about 1 psi above ambient atmosphericpressure to about 25 psi above ambient atmospheric pressure.
 21. Themethod of claim 13 wherein said pressurizing is to a pressure of about 6psi above ambient atmospheric pressure.
 22. The method of claim 13wherein said cooling occurring within a range of rates of about 0.5degrees Celsius per minute to about 40 degrees Celsius per minute. 23.The method of claim 13 wherein said annealing further comprises coolingat a rate of about 20 degree Celsius per minute.
 24. A method forannealing lithium tantalate (LiTaO₃) structures, the method comprising:heating a lithium tantalate structure in gas and lithium tantalatepowder environment to a maximum anneal temperature; pressurizing thesealed gas atmosphere to exceed ambient atmospheric pressure;maintaining temperature and pressure for a period of at least about 4hours; and cooling the structure to ambient temperature, wherein saidpressurizing is to within a pressure range of about 1 psi above ambientatmospheric pressure to about 25 psi above ambient atmospheric pressure.25. A method for annealing lithium tantalate (LiTaO₃) structures, themethod comprising: heating a lithium tantalate structure in gas andlithium tantalate powder environment to a maximum anneal temperature;pressurizing the sealed gas atmosphere to exceed ambient atmosphericpressure; maintaining temperature and pressure for a period of at leastabout 4 hours; and cooling the structure to ambient temperature, whereinsaid pressurizing is to a pressure of about 6 psi above ambientatmospheric pressure.
 26. A method for annealing lithium tantalate(LiTaO₃) structures, the method comprising: heating a lithium tantalatestructure in gas and lithium tantalate powder environment to a maximumanneal temperature; pressurizing the sealed gas atmosphere to exceedambient atmospheric pressure; maintaining temperature and pressure for aperiod of at least about 4 hours; and cooling the structure to ambienttemperature, wherein said cooling occurring within a range of rates ofabout 0.5 degrees Celsius per minute to about 40 degrees Celsius perminute.
 27. A method for annealing lithium tantalate (LiTaO₃)structures, the method comprising: heating a lithium tantalate structurein gas and lithium tantalate powder environment to a maximum annealtemperature; pressurizing the sealed gas atmosphere to exceed ambientatmospheric pressure; maintaining temperature and pressure for a periodof at least about 4 hours; and cooling the structure to ambienttemperature, wherein said annealing further comprises cooling at a rateof about 20 degree Celsius per minute.
 28. A method for annealing acrystalline substrate having the formula RMO₃ where R is an alkalineearth metal, M is a Group IVB or Group VB metal and O is oxygen, themethod comprising: heating said crystalline substrate in an environmenthaving powder also formed of RMO₃, the powder being disposed in theenvironment, the crystalline substrate being separated from the powderby a barrier, the barrier inhibiting movement of the powder thereacrosswhile permitting a free flow of gas within the environment.
 29. A methodfor annealing a crystalline substrate, the method comprising: providinga container; placing the crystalline substrate in a first region of thecontainer; placing a powder in a second region of the container, thepowder being of the same composition as the crystalline substrate;separating the first region and the second region with a barrier, thebarrier inhibiting movement of the powder thereacross while permitting afree flow of gas within the container; and heating the container. 30.The method of claim 29 wherein the barrier is a porous wall.
 31. Themethod of claim 29 wherein the crystalline substrate and the powder arelithium niobate (LiNbO₃).
 32. The method of claim 29 wherein thecrystalline substrate and the powder are lithium tantalate (LiTaO₃). 33.The method of claim 29 further comprising; pressurizing the containerwith an oxygen gas environment during said heating.